Numerous cephalosporin antibiotic compounds having a heterocyclic ring bonded to a sulfur atom attached to the 3-methylene group of the cephalosporin ring system are known. For example, Ryan in U.S. Pat. No. 3,641,021 describes cephalosporin antibiotics substituted in the 3-position with the 1H-tetrazole-5-ylthiomethyl and 1,3,4-thiadiazole-2-ylthiomethyl groups and in the 7-position with the D-phenylglycylamido and D-mandelamido groups. For example, 7-D-mandelamido-3-(5-methyl-1,3,4-thiadiazole-2-ylthiomethyl)-3-cephem-4-c arboxylic acid is described by Ryan.
Tokano et al., U.S. Pat. No. 3,516,997, describe other cephalosporin antibiotics having the tetrazole-5-ylthiomethyl, substituted tetrazole-5-ylthiomethyl, 1,3,4-thiadiazole-2-ylthiomethyl, and 1,3,4-oxadiazole-2-ylthiomethyl group in the 3-position.
Further, Crast in U.S. Pat. No. 3,757,014 describes cephalosporin compounds substituted in the 3-position with the 5-hydroxymethyl-1,3,4-oxadiazole-2-ylthiomethyl group. Cephalosporins substituted in the 3'-position with the 2-methyltetrazole-5-ylthiomethyl group are described in U.S. Pat. No. 3,757,015.
The cephalosporin antibiotics described in the aforementioned patents were prepared by reacting 7-aminocephalosporanic acid (7-ACA) with the appropriate heterocyclic thiol. The reaction proceeds via the nucleophilic displacement of the acetoxy group of 7-ACA or of an amino-protected 7-ACA at or near neutral pH to provide the 7-amino-3-heterocyclic-thiomethyl-3-cephem-4-carboxylic acid. For example, 7-ACA is reacted as described in U.S. Pat. 3,641,021 with 5-methyl-1,3,4-thiadiazole-2-thiol to yield 7-amino-3-(5-methyl-1,3,4-thiadiazol-2-ylthiomethyl)-3-cephem-4-carboxylic acid. The 3-thio-substituted nucleus is then N-acylated in the 7-position with the desired acyl derivative of a carboxylic acid to afford the described antibiotics.
The described 7-amino-3-heterocyclic-thiomethyl-3-cephem-4-carboxylic acids are valuable intermediates in the preparation of cephalosporin antibiotics, for example, those described in the above-cited Ryan and Tokano patents. Particularly noteworthy are the antibiotics cefamandole, 7-D-mandelamido-3-(1-methyl-1H-tetrazole-5-ylthiomethyl)-3-cephem-4-carbox ylic acid, described by Ryan in U.S. Pat. No. 3,641,021 and the commercial antibiotic, cefazolin, 7-(1H-tetrazole-1-yl)acetamido- 3-(5-methyl-1,3,4-thiadiazole-2-ylthiomethyl)-3-cephem-4-carboxylic acid, described by Tokano et al., U.S. Pat. No. 3,516,997.
7-Aminocephalosporanic acid (7-ACA), is a well known cephalosporin nucleus intermediate used in the preparation of many cephalosporin antibiotics, For example, N-acylation of 7-ACA with thiophene-2-acetyl chloride affords the commercial antiobiotic cephalothin.
7-ACA is produced by the N-deacylation of the .alpha.-aminoadipamido side chain of cephalosporin C. Cephalosporin C is produced by culturing Cephalosporium acremonium or other species of cephalosporium in nutrient media. The N-deacylation of cephalosporin C is carried out with a phosphorus halide, e.g., phosphorus pentachloride, to generate the intermediate imino halide of the side chain. The imino halide is converted to an imino ether which is decomposed to effect the side chain cleavage and provide the cephalosporin nucleus, 7-ACA.
The impurities associated with cephalosporin C of fermentation origin can carry through the N-deacylation reaction and complicate the purification of the 7-ACA product. Impurities of fermentation origin such as polysaccharides, aminoacids and other present difficult isolation and purification problems in the large scale manufacture of 7-ACA of suitable purity for use as an intermediate for antibiotics.
The preparation of the intermediate 7-amino-3-heterocyclicthiomethyl-3-cephem-4-carboxylic acids (hereinafter referred to as 3-substituted nucleus) by the nucleophilic displacement of the acetoxy group of 7-ACA or an N-protected 7-ACA with the heterocyclic thiol is accompanied by the formation of highly colored impurities. These impurities render difficult the purification of the 3-substituted nucleus. Accordingly, in order to obtain the final antibiotic in pharmaceutical purity, either the nucleus is purified by repeated isoelectric precipitation or the final antibiotic product obtained by the acylation of the unpurified 3-substituted nucleus is purified. In the large scale commercial preparation of antibiotics cumbersome and expensive purification procedures are undesirable and uneconomical. Accordingly there is a need in the cephalosporin art for a convenient method for the purification of 7-ACA and the 3-thio-substituted nucleus compounds which avoids multiple recrystallization to achieve pharmaceutical purity.